The present invention relates to a method and a circuit for the generation of a speed reference signal adaptable to variations in the motion of a vehicle for use in an antiskid system for fluid-operated vehicular brakes.
Because it is difficult and, for reasons of cost, hardly economically feasible to use a signal required for antiskid systems exactly corresponding to the true vehicle speed as a speed reference signal, the vehicle wheels' instantaneous velocity signals which are available anyway are conventionally used for the derivation of a speed reference signal required to determine the slip of the respective wheels. Depending on the driving condition, this derived speed reference signal may differ substantially from the true vehicle speed.
For the purpose of obtaining a speed reference signal suited for slip control, it is already known to release fully the braking force at one or several vehicle wheels intermittently, so that these wheels are allowed to rotate freely for a short period of time, whereby the instantaneous velocity signal provided by the wheels in this period is a relatively accurate measure of the true vehicle speed, provided that the non-driven wheels are employed. However, in this known method the braking effectiveness obtainable with these wheels is so considerably reduced that the optimum or maximum possible braking values are far from being attained.
The antiskid systems for fluid-operated vehicle brakes, it is also known to initiate a control cycle causing a transient pressure decrease in a vehicle brake either dependent on what is termed a "deceleration criterion", or dependent on what is termed a "slip criterion". In addition to circuit configurations employing either the first or second criterion, an antiskid system is also known from German Patent DE-AS No. 1,937,123, for example, in which both criteria are employed.
In the system of the above-cited German Patent, fixed response thresholds are preset for the "deceleration" or "acceleration" criterion. If these thresholds are passed by a voltage derived from the wheel rotational speed and proportional to the wheel acceleration, a control cycle is initiated or terminated for the wheel concerned. However, in view of the fact that uneven road surfaces or variations in the rolling radius, for example, may cause quite considerable accelerations or decelerations at the individual vehicle wheels, the respective response thresholds must be set at a level sufficiently high to avoid spurious initiation or termination of control cycles.
On the other hand, such high response thresholds increase the danger of wheel lock without deceleration values being thereby attained which cause the initiation of a control cycle. In such a case, the so-called "slip criterion" will take effect in which, as an alternative to the "deceleration criterion", a control cycle will be initiated if the wheel speed drops below the vehicle speed a predetermined amount.
Here, too, the difficulty is to generate a speed reference signal representing the vehicle speed with a sufficient amount of accuracy for comparison with the instantaneous velocity signals of the individual wheel to determine their respective slip.
In the device described in the above-cited German Patent, the instantaneous velocity signal of the fastest rotating vehicle wheel is used as the speed reference signal for slip control. As a result, in cases where a motor vehicle equipped with such an antiskid system is to start on slippery ground, for example on snow, and the driven wheels spin, the instantaneous velocity signal of the highest spinning wheel, which signal is out of all defined proportion to the true vehicle speed, is used as the speed reference signal. Consequently, an unrealistically high slip is simulated for the non-driven wheels and the driven wheel which spins less, which may result in a dangerous release of the braking force at these wheels.